Everything works fine, until it is neeed to post the invoice to general ledger. The system shows an error text that it is impossible to post as "date is not in specified accounting date". In tutorials, the date is set 07/12/2012, i have set the same date, but error shows up.

I have tried todays date and dates before, but then "post to general ledger" action dissapears on the action list.

the problem is a still open but ended Accounting Period in the Financial Information Tab of your company (or VIFIB). You can stop this accounting period and start a new one somwhere in the past and with a stop date in the future.